arcpy.mapping is a module of ArcPy which the Python site package for ArcGIS.
arcpy.mapping is a Python scripting module that is part of the ArcPy site package. It gets installed with ArcGIS for Desktop and is available to all licenses. It was designed primarily to manipulate the contents of existing map documents (.mxd) and layer files (.lyr). It also provides functions to automate exporting and printing. Arcpy.mapping can be used to automate map production; it extends the capabilities of Data Driven Pages and is required to build complete map books because it includes functions to export to, create, and manage PDF documents. Finally, arcpy.mapping scripts can be published as geoprocessing services and the script functionality can be made available to web applications.
Source: ArcGIS Help 10.1.